A data storage system stores and retrieves information on behalf of one or more external host computers. A typical data storage system includes power supply circuitry, a storage processing device, and a set of disk drives. The power supply circuitry provides power to the storage processing device, thus enabling the storage processing device to load data into and retrieve data from the set of disk drives on behalf of the external host computers.
Typically, the storage processing device includes several sub-components, called field replaceable units (“FRUs”), for performing various functions of the storage processing device. These FRUs are designed and integrated into the storage processing device in a way that enables them to be replaced at the location of the data storage system, thereby eliminating the need to move the entire data storage system to a repair facility to perform the replacement of the failed sub-component. These FRUs can include a CPU module, I/O devices, memory modules and power supply modules. Typically, one of the I/O modules controls communications between the CPU module and the set of disk drives and another I/O module controls communications between the CPU module and the external host computers. In the event that one of the I/O module FRUs is removed from the storage processing device, the operation of the storage processing device should be ceased, so that the I/O module can be replaced, to minimize the chances of any data being lost.